1. Field of Invention
This invention relates to a method of determining a timing offset between a first clock and a second clock in a communications network.
2. Background of the Invention
It is standard for a device connected to a computer network to have an internal clock for time keeping purposes. Typical hardware clocks provide millisecond resolution and timing. Usually, the internal clocks of two or more devices communicating over a network run independently of each other and thus may not be synchronised. For certain types of network communication, it is desirable that the timing offset between the unsynchronised internal clocks of the two or more communicating devices be known or be predictable. For example, measuring network delay in an IP network is simplified if the timing offset between the clocks of the communicating devices is known.
Network delay in an IP network is a measure of how long it takes for a packet to get from one point in the network to another. Network delay can be measured either for a packet round trip, or in a single direction.
One method for measuring round trip delay involves using the well-known network ‘ping’ feature. The ‘ping’ feature echoes back a packet from its reception point to the point in the network where the packet was initially transmitted. The round trip delay time is simply calculated as the difference between the time the packet was received back at its starting point and the time the packet was initially sent from its starting point. Since this calculation is reliant only on the timing of the clock at the starting point there is no need for the starting point clock and the reception point clock to be synchronised.
One known method for measuring single direction network delay relies on an absolute clock to synchronise the timing of the clock at the network starting point and the timing of the clock at the network end point. Provided the clocks at the network start and end points are synchronised to an absolute clock, single direction delay is simply measured as the difference between the time a packet is received at the endpoint and the time the packet is sent from the starting point. The clocks at the start and end points may for example be synchronised to a Global Positioning System (GPS) time receiver or any other such source of absolute time measurement.
RFC 1305 describes The Network Time Protocol (NTP). The NTP is designed to distribute time information from an absolute time source to devices in a large network such as the Internet.
Jitter is the amount that packet transmission rate varies from the mean during a current time period. Jitter measurements assume a set interval between packets being sent as part of the test. Thus jitter measurements only give an indication of delay effects by a comparison of the measured interval between two packets being received and the set packet transmission interval. Jitter measurements do not give the single direction delays experienced by individual packets.
It is desirable to be able to determine or predict the timing offset between two clocks in a computer network. This would have many advantages, including allowing one way network delay to be determined without having to use an absolute clock to synchronise the clocks at the network transmission and reception points.